1. Field of the Invention
The present invention relates to a process for the preparation of esters from alcohols and acids by means of liquid-phase equilibrium reactions on ion exchangers, which process is carried out in a prereactor and in a rectifying column with external reactors.
2. Discussion of the Background
Since esterification is not a very one-sided equilibrium reaction, special measures, as for example, the addition of excess starting compound or a multi-step reaction, are taken in order to increase the conversion of valuable feedstocks and to obtain the esters in maximum yield and purity.
Thus, esterification on cation exchangers is carried out, according to DE-C-1,768,104 in a precolumn and a subsequent reaction distillation column.
Due to the high thermal load of ion exchangers in a reaction distillation column and due to the difficult separation by means of distillation in the presence of ion exchangers, esterification and rectification are spacially separated in other processes. In DE-C-3,121,383, esterification is carried out in a reactor on a cation exchanger. In a subsequent cation exchanger-free distillation column, the reaction mixture is separated. By recycling unconverted starting compounds into the reactor, high yields of ester can be obtained.
H. Schoenmakers and W. Buhler, Chem.-Ing.-Tech., Vol. 54, p. 153 (1982), disclose another possibility of esterification. They use an apparatus, which comprises a prereactor and a distillation column with external reactors. The esterification is catalyzed by ion exchangers in the reactors. Thus, the alcohol-acid mixture is guided first into the prereactor. The reaction mixture is fed into the distillation column. The liquid phase draining into the column is led out multiple times on the side, directed through an external reactor and returned from there to the next lower plate of the column.
With equimolar quantities of alcohol and acid or with a slight excess of alcohol quantities, conversions ranging from about 55 to 60% are obtained. Naturally, the process avoids recycling unconverted starting compounds into the prereactor, but the conversions for commercial use are very low.